What are the roles of zonula occludens?

• A. They prevent migration between apical and lateral membrane domains and are involved in intracellular signaling.
• B. They anchor cells to the basement membrane.
• C. They promote diffusion between apical and lateral membranes.
• D. They produce extracellular matrix.

Answer: (A)

Zonula occludens, or tight junctions, create a seal at the apical border between neighboring epithelial cells. Their main roles are to prevent movement of membrane components between the apical and lateral (basolateral) surfaces, helping to keep these membrane domains distinct and preserving cell polarity. This “fence” function also contributes to controlling what passes paracellularly between cells, shaping how ions and solutes move across an epithelium.

In addition, tight junctions act as a signaling hub. They connect transmembrane proteins like occludin and claudins to scaffold proteins (such as ZO-1, ZO-2, and ZO-3) that link to the actin cytoskeleton, influencing intracellular signaling pathways and cytoskeletal organization. This means they don’t just form a barrier; they help regulate cellular behavior in response to environmental cues.

The other statements don’t fit because anchoring cells to the basement membrane is the job of hemidesmosomes, not tight junctions; producing extracellular matrix is typically a function of other cell types like fibroblasts; and tight junctions do not promote diffusion—they restrict it to maintain polarity and barrier function.